1. Field of the Invention
This invention relates to the measurement of the mutual phase relationship of the spectral components generated by a signal generator. Such a measurement is necessary if one wants to use the signal generator as an harmonic phase reference generator. An harmonic phase reference generator is a repetitive electrical pulse generator with an adjustable repetition rate, also called the fundamental frequency. An harmonic phase reference generator is used as the harmonic phase standard for a “large-signal network analyzer”, also known as LSNA. An LSNA, typically operating in the microwave frequency range, measures time varying electrical quantities like the current waveforms and voltage waveforms at the terminals of an electrical device like for example a diode or a transistor. The characterization of an harmonic phase reference generator itself is time consuming and can take more than a whole day. An harmonic phase reference generator is completely characterized when one knows its output reflection coefficient as a function of frequency across its full bandwidth of operation and when one knows the amplitude of and the mutual phase relationship between all the spectral components which are generated by the harmonic phase reference generator, and this for a multitude of fundamental frequencies.
2. Description of the Related Art
Many prior art methods exist to measure the phase difference between two spectral components that have exactly the same frequency, as it is described e.g. by Anderson (U.S. 2004/0113632 A1). Such a phase measurement is typically performed by a network analyzer, which measures the phase difference between the signal that is incident to a device-under-test and the signal, at exactly the same frequency, that is transmitted or reflected by the device-under-test. Many methods also exist to measure the amplitudes of a set of spectral components that are generated by a signal generator, as described by Gorin et al. (U.S. Pat. No. 6,980,915 B2). Such an amplitude measurement is typically performed by a spectrum analyzer. None of the abovementioned prior art methods can be used, however, to measure the mutual phase relationship of a set of spectral components that are generated by one signal generator. This type of phase measurement is technically difficult because all of the spectral components that are subject to the phase measurements have a different frequency. The measurement of the mutual phase relationship of a set of spectral components that are generated by a signal generator is necessary for characterizing so-called harmonic phase reference generators that are used for calibrating a large-signal network analyzer (LSNA), a new microwave instrument that was commercially introduced in 2003. The harmonic phase reference generator is used to eliminate the phase distortion error of the instrument.
In the prior art only one method is described for the characterization of an harmonic phase reference generator. The method is explained in the following. The output reflection coefficient of the harmonic phase reference generator is measured by a vector network analyzer. Next a microwave sampling oscilloscope is characterized by using the nose-to-nose calibration method as described in “Individual Characterization of Broadband Sampling Oscilloscopes with a ‘Nose-to-Nose’ Calibration Procedure”, by Jan Verspecht and Ken Rush, published in IEEE Transactions on Instrumentation and Measurement, Vol. 43, No. 2, pp. 347-354, April 1994. The harmonic phase reference generator to be characterized is then connected to the input of the oscilloscope and the repetitive electrical pulse which appears at the output of the harmonic phase reference generator is being measured and digitized by the oscilloscope. This process is illustrated in FIG. 1. After being digitized by the oscilloscope the measured waveforms are processed by a computer that calculates the phase and amplitude of all spectral components generated by the harmonic phase reference generator. The spectral components are calculated using the discrete Fourier transform algorithm. The whole process is explained in “Large-Signal Network Analysis—‘Going beyond S-parameters’” by Jan Verspecht, 62nd ARFTG Conference Short Course Notes, USA, December 2003. The method explained above is the only method I know of that is used to characterize harmonic phase reference generators and has significant disadvantages. A first disadvantage is that the method requires a measurement lab which is equipped with at least three microwave sampling oscilloscope modules and two oscilloscope mainframes in order to be able to perform a ‘nose-to-nose’ calibration procedure. A second disadvantage of the method is that it takes a long time to perform. Even after one has performed a nose-to-nose calibration, which by itself takes one day, it still takes at least several hours for the characterization of one harmonic phase reference generator for a multitude of fundamental frequencies. This is a real problem when a whole series of harmonic phase reference generators needs to be calibrated. With the invented method one does not need sampling oscilloscopes and the characterization of one harmonic phase reference generator is possible in less than 15 minutes.